[Catapult] Refactor BUG= presubmit check
[Catapult] Refactor BUG= presubmit check

This CL changes the BUG= presubmit check to allow Chromium issues,
as long as they are prefixed by chromium:.

BUG=catapult:#2006

Review URL: https://codereview.chromium.org/1691883002

catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,SummerLW/Perf-Insight-Report,benschmaus/catapult,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report,sahiljain/catapult,SummerLW/Perf-Insight-Report,benschmaus/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,SummerLW/Perf-Insight-Report,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report